The present invention is directed to glass concentrate capsules which comprise a plurality of strands of glass fibers encapsulated in a collimated array within a low molecular weight crystalline organic compound, to a process of manufacture of such capsules, to thermoplastic resins reinforced with such capsules and to a process of reinforcement of thermoplastic resins by coextrusion with such capsules.
The use of glass fibers as a reinforcing medium in thermoplastic resin composites is well known in the prior art.
In the preparation of glass fiber reinforced composites it is conventional to use segments of glass fiber strands which vary in length from 0.80 to 20 mm or longer. These glass strands are conventionally made up from 200 to 800 filaments having a diameter in the order of 8 to 15 microns arranged in a parallel configuration. The surface of the glass strand is conventionally coated with a coupling agent and a film forming size which keeps the microfibers together and maintains the integrity of the strand. The glass strands are chopped into segments of uniform length and are then dry blended with a thermoplastic resin matrix and fed to an extruder or injection molding machine wherein the fibers are distributed throughout the resin matrix and serve as reinforcing elements in the finished composite.
Dry-blending of the glass and resin matrix is considered to be the simplest, most versatile and economical route for large volume preparation of composites. However, such dry-blending involves high capital investment for the equipment necessary to avoid the very severe problems of non-uniformity of glass distribution and segregation, debundling, bridging, haystacking and matting of glass during blending, feeding and processing. Consequently, special vibrator proportioning and metering feeds are required.
Debundling is the term used to describe that occurrence where the glass strand loses it intregity and the individual microfibers are scattered. The loose microfibers can undergo bridging or form haystacking configurations (haystacking) in the hopper which feeds the extruder or injection molding machine. As a result of this haystacking, the desired feed ratio of glass fibers to resin matrix in the molding or extrusion operation can be upset and an inferior product is produced. Alternatively, the haystack can be fed to the machine in the nature of an embolus and causes matting of the glass fibers, clogging of the machinery and production of inferior non-uniform products.
Attempts to solve the problems of debundling and haystacking led researchers to coat the glass strand with thermoplastic resin polymers. Bradt, in U.S. Pat. No. 2,877,501, teaches coating the outside of an endless glass strand with a polymeric coating followed by heat treatment to fuse the polymer then cutting the strand to the desired length. In this method, the cut ends of the glass strands contain exposed ends of microfibers and are possible sites for subsequent debundling. Moreover, there is a polymer gradient which decreases toward the center of the strand. Consequently, the individual microfibers in the core of the strand may not be coated with polymer. Thus, the shearing forces of injection molding or extrusion could cause abrasion of the individual microfibers in the core of the strand with resulting damage to these fibers which detracts from their reinforcing ability.
Malinowski et al. in U.S. Pat. No. 2,688,774, Herman et al. in U.S. Pat. No. 3,265,644; and Wiczer in U.S. Pat. No. 3,278,329 provide a partial solution to the problems mentioned above by coating the glass strand with monomer followed by in situ polymerization to give a single glass strand contained within a thermoplastic resin capsule. In some instances the monomers wet the individual microfibers and upon polymerization provide a coating which helps to protect the microfibers from the adverse effects of abrasion during composite preparation.
However, in the foregoing methods, it is not possible to obtain a high concentration of glass fibers in capsule form which is a desired feature in the preparation of glass filled composites. Moreover, in many instances the capsules of the prior art, which contain a single glass strand, rather than a plurality of collimated glass strands, have a different density, size and shape than the particles of resin matrix being fed to the extruder or molding apparatus. These differences may result in segregation of the respective particles and a non-uniform product.
Baer, in U.S. Pat. No. 3,671,384 discloses a process of manufacture of glass concentrate capsules which contain a high concentration of glass strands arranged in a collimated array encapsulated with a thermoplastic vinyl polymer. These capsules can be used for reinforcement of thermoplastic resin matrices of homo- and copolymers of vinyl chloride and homo- and copolymers of styrene. The desired encapsulation is obtained by: wetting the strands with vinyl monomers, dispersing them in an aqueous medium containing a suspending agent, and polymerizing the monomers. The fluidity of the monomers permits the stacking of the strands to build capsules by interfacial and hydrodynamic forces. The capsules preserve their integrity while going through the transition from fluid monomers, to viscous syrup, to solid polymers. A problem with vinyl polymers as encapsulating materials is that these polymers are not molecularly dispersible in some matrices (for example, polyesters, nylons, polycarbonates). Hence, when they are used for reinforcement of such matrices damage to some mechanial properties can occur. A further problem exists in the presence of suspending agent which can interact with the thermoplastic matrix and can cause impairment of certain physical and mechanical properties. A further problem exists in the extended period of time required to effect encapsulation.